1. Field of the Invention
The present invention relates to a solid-state imaging device and a driving method therefor. More particularly, the present invention relates to a solid-state imaging device such as a CCD (Charge-Coupled Device) imaging device for transducing incident light into signal charges and horizontally transferring the charges to output an image signal through an output amplifier. The invention also more specifically relates to a driving method of driving the imaging device for horizontal transfer.
2. Description of the Background Art
Japanese patent laid-open publication No. 50409/1995 discloses bifurcating a sole transfer path, that is, a shift register, and providing an amplifier at the distal end of each bifurcated transfer path. Each amplifier is a floating diffusion amplifier (FDA) and differs from one another in charge detection sensitivity or charge-to-voltage conversion efficiency. When a subject being imaged is of lower luminance, the higher-efficiency amplifier outputs the output signal. This amplifier selection improves the voltage conversion efficiency for lower signal charges obtained on light reception, and raises the sensitivity. When the subject being imaged is of higher luminance, the lower-efficiency amplifier outputs the output signal. With the use of this amplifier, the image generated by the output signal is of a wider dynamic range. Another Japanese patent laid-open publication No. 298626/1996 is substantially similar to the '409 publication.
Still another Japanese patent laid-open publication No. 244340/1993 also teaches bifurcating a sole transfer path, or a shift register. However, the signal charges obtained on light reception or optical sensing are alternately switched at the bifurcating section and transferred to the respective amplifiers. The frequency of the driving pulse supplied to the transfer path not bifurcated is twice as high as the driving pulse supplied to the bifurcated transfer paths. In other words, a driving frequency one-half the usual driving frequency suffices for driving the bifurcated transfer paths. Thus, the amplifier renders it possible to raise the transfer speed as the driving frequency is maintained within the frequency band prescribed as its operating characteristics.
In the '409 and '626 publications, there is simply disclosed guiding the signal charges to a selected transfer path. The '340 publication discloses alternately outputting signal charges. If those publications are simply combined together, it would indeed be possible to output images that satisfy the requirements for high sensitivity and wide dynamic range.
In producing a color image, color attributes are allocated to the respective signal charges. However, there is neither suggestion nor disclosure as to how the signal charges are transferred and distributed to the bifurcated transfer paths from color to color. Thus, with those prior art publications combined together, it would not be possible to provide an image reduced in noise based on a white-balance gain with respect to color.
If with the conventional solid state imaging device a subject with lower color temperature, for example, is shot, then the amounts of signal charges obtained in the photosensitive cells of the imaging device are larger for red (R) pixels and smaller for blue (B) pixels. If the signal charge are horizontally transferred from the transfer path not bifurcated in the sequence of an R pixel, a first green (G1) pixel, a B pixel and a second green (G2) pixel, the amount of charges left over by the forward R pixel and mixed into the rear side G1 pixel is greater than the amount of charge mixing between the forward side B pixel and the rear side G2 pixel. Thus, the G1 and G2 pixels which are of the same color signal differ in the signal quantity, thus affecting the finished image as a fixed pattern noise.
In addition, deterioration in the transfer efficiency of signal charges is caused with the solid state imaging device in which signal charges are not branched optimally at a branching electrode such that signal charges to be sent to one of the horizontal transfer paths are intruded into the other transfer path.
More specifically, such a case is now considered in which signal charges are transferred from a sole horizontal transfer path in the sequence of a G1 pixel, an R pixel, a G2 pixel and a B pixel, and are branched at the branching electrode, signal charges of the pixels G1 and G2 are sent to one of the horizontal transfer paths and those of the pixels R and B are sent to the other transfer path. If the ambient temperature at the time of imaging is low, part of signal charges of the pixel G1 is mixed into signal charges of the pixel R.
Such deterioration in the transfer efficiency, i.e., transfer deterioration, of signal charges is caused not only when the ambient temperature at the time of imaging is low, but also when the color temperature of a subject being imaged is low or the ISO (International Organization for Standardization) sensitivity is high. In particular, in case of a subject with a low color temperature, signal charges are mixed in different quantities, even if the signal charges are of the same color. More specifically, supposing that signal charges of pixels G1, R, G2 and B obtained on imaging a subject of a low color temperature are transferred in that order, the signal charges are mixed in the pixel R from the pixel G1 in larger quantity than the quantity of the signal charges mixed from the pixel G2 into the pixel B. The result is that signal quantities of the pixels R and B become different from each other, with the difference in signal quantity then being visualized as noise in the image.
In the meantime, in those prior art publications, there is neither suggestion nor disclosure as to the sequence of readout of color signals or as to how signal charges are to be split to the branched transfer paths depending on the colors. It is noted that, if the branching section receives signals of different colors, and cannot correctly transfer the signal charges to the branches, part of those signals of different colors may be mixed with each other on the branches.